Hesitation/Misfire Under Acceleration

[Sir Paolo]

Hi All,
My 98 Chim 400 has developed a misfire under hard acceleration after about 4.5k rpm.

I have checked HT leads, fitted new cap and rotor, plus a coil (which was no doubt the original one).

Since doing this, the problem is still there, but less pronounced and is more intermittent.
For example, it doesn’t seem to do it so much in 2nd, but more so in 4th gear.

Similar threads have described this as similar to hitting the rev limiter, as it bogs down and is reluctant to rev further.

Up to that point, the car pulls very well.

I’m planning on running the engine in the dark to look for any shorting, but that sums up my position really- being in “the dark”, when it comes to electrics.

Although I did check the voltage at the fuel pump- it’s down at 11.55v, but no doubt it’s been like that for ages.
As such, it maybe a contributing factor, if not the cause.

I have a Rover Gauge, but don’t really know what the readings mean, and as it only happens under load, I’m thinking that maybe a session on a rolling road might be in order.

Any advice, much appreciated as always.

Cheers guys!

[Belle427]

I’d still be concentrating on the ignition system, if you still run extenders then it’s worth ditching them.
Aftermarket caps and rotors etc can be sketchy, sadly quality parts are no longer available.
What ht leads are you using?

[Sir Paolo]

Hi, thanks for your response.
I have ditched the extenders, and I’m running NGK Iridium plugs which are only about 6k miles old (BCP6EIX - IIRC).

The HT leads are also about the same age, and are 8mm silicone from Powerspark.
The new rotor arm is a “red” from them too.
Plus the new coil is a proper Bosch version.

All LT cables seem to check out ok, but that’s where the planned running in the dark comes in, just to double check.

And yes, I would agree that it’s spark related, although happy to be proved wrong.

[Belle427]

I’ve heard the power spark stuff is rubbish sadly so I’d certainly start with the rotor and cap.
Where you buy from is another matter, if you still have original parts I’d try them first.
Distributor Doctor is supposed to be good but not cheap.
Best of luck anyway, these are the worst annoying problems!

[Sir Paolo]

It’s interesting that you’ve heard that Powerspark stuff is rubbish.
All I can say is since fitting their cap and rotor things have improved (although that may also be due to the new coil fitted at the same time).

As I say, I’m not the best with electrical diagnostics which is why I’m thinking of an RR session.

Cheers
Paul

[Belle427]

Not sure where you are but maybe worth booking into kits and classics as I believe he carries decent spares from doing management conversions.

[NMNeil]

I'd start with the free stuff.
Pull off the vacuum line to the fuel pressure regulator and see if you can smell petrol. If you can it's leaking internally and can cause the symptoms you describe.

[Sir Paolo]

I’m in Devon, so unfortunately Chesterfield is a bit of a trek (although a reasonable distance would definitely be worth it).

Good point re. regulator, I’ll check out.

Many thanks, both of you.

[Arnold Cunningham]

On a Rover V8 with a misfire under load (and idle is running properly on all 8), I always start with a set of leads and a distributor cap.

[Super Sonic]

Also check the engines Earth strap.

[Sir Paolo]

NMNeil said:

I'd start with the free stuff.
Pull off the vacuum line to the fuel pressure regulator and see if you can smell petrol. If you can it's leaking internally and can cause the symptoms you describe.

Hi, I think you maybe onto something here.

I pulled off the the vacuum line to the regulator and there’s definitely a smell of petrol.

The pipe also fell off the plenum end, too without any persuasion.

I take it that could point to a defective diaphragm?

[NMNeil]

Sir Paolo said:

Hi, I think you maybe onto something here.

I pulled off the the vacuum line to the regulator and there’s definitely a smell of petrol.

The pipe also fell off the plenum end, too without any persuasion.

I take it that could point to a defective diaphragm?

Looks like you found at least one of the culprits.
Yes, diaphragm leaks are very common, but it's easy and cheap to replace.

[Sir Paolo]

Super Sonic said:

Also check the engines Earth strap.

Thanks- this is secure, and I have also added an extra earth cable from the block to the battery -‘ive.

Anything to help with the marginal earths, that after many years are probably compromised now.

[Sir Paolo]

NMNeil said:

Looks like you found at least one of the culprits.
Yes, diaphragm leaks are very common, but it's easy and cheap to replace.

I guess that with the fuel pump getting only 11.55 volts, it wouldn’t take much loss of pressure at the fuel rail to add to the issue.

I’ll get the fuel pressure tested at the Schrsder valve before finally condemning the regulator.

Cheers
Paul

[NMNeil]

Sir Paolo said:

I guess that with the fuel pump getting only 11.55 volts, it wouldn’t take much loss of pressure at the fuel rail to add to the issue.

I’ll get the fuel pressure tested at the Schrsder valve before finally condemning the regulator.

Cheers
Paul

It's not just the pressure. If the diaphragm is leaking the vacuum line will be sucking raw petrol into the intake manifold giving the ECU a real workout as it tries to compensate.
But connect a pressure gauge, turn on the ignition to prime the pump and when the prime is over the pressure should stay where it is. If it drops quickly it's either a bad regulator or an injector stuck open.

[Sir Paolo]

As I don’t have a fuel pressure gauge, I might just as well get a new pressure regulator anyway.

I’d end up spending at least £30 on a tester, which is almost half the cost of a regulator - and given the fact that there is definitely a strong smell of petrol in the vacuum pipe, a tester might only end up confirming what I already suspect.

I believe there’s enough evidence against the regulator for the jury to convict.

[Steve_D]

I've not experienced a failed regulator diaphram but logic says if there is a hole/split in the diaphram then fuel will come out of that pipe when you turn on the fuel pump so a pretty straight forward test.

Steve

[Sir Paolo]

Steve_D said:

I've not experienced a failed regulator diaphram but logic says if there is a hole/split in the diaphram then fuel will come out of that pipe when you turn on the fuel pump so a pretty straight forward test.

Steve

Hi Steve,
It’s worth checking as you suggest, but the smell of petrol at the regulator end of the vacuum pipe was unmistakable.

Do I take it that if the regulator diaphragm was undamaged then there shouldn’t be any petrol fumes (which could indicate a leak) at all?

Thanks
Paul

[keynsham]

Does it happen when the engine is cold, or perhaps not fully warmed up? I had ther same issue witha Chimaera I owned many years ago and it was the lamda sensor in the exhaust. The lamda sensor input it ignored when the car is cold as the mixture is rich anyway so the problem doesnt become apparent until the engine is warm.

[Sir Paolo]

keynsham said:

Does it happen when the engine is cold, or perhaps not fully warmed up? I had ther same issue witha Chimaera I owned many years ago and it was the lamda sensor in the exhaust. The lamda sensor input it ignored when the car is cold as the mixture is rich anyway so the problem doesnt become apparent until the engine is warm.

Hi, thanks for your response.

I don’t know, as I never rev the engine beyond 3k until it is properly warmed up.
It does only do it under increasing loads, I.e. it won’t do it in neutral or 1st/2nd gear so much.